Photolithographic photomasks are critical for the precise transfer and fabrication of shapes onto a semiconductor wafer. Such photomasks are used repeatedly in a photolithographic exposure tool to produce many wafers.
Over time, the photomasks may develop defects. This may be due to oxide growth of the photomasking films, physical degradation due to optical and environmental exposure, chemical contamination, chemical gettering, handling or any combination of the aforementioned activities.
To prevent the fabrication of defective wafers resulting from a defective or degraded photomask, either the photomask or the wafers need to be inspected or monitored. To do this, there are many techniques known in the art.
One such technique is the periodic inspection of the photomasks after, say, several hundred wafer passes. The problem with this technique is that the photomask inspection requires high-sensitivity inspection tools which are usually maintained in the photomask fabrication facility and not usually operated in the semiconductor production environment.
Another technique is the monitoring of the wafers themselves. One way to do that is by correlating photo limited yield (PLY) data with defective or degraded photomasks. Another way would be to look at the product using a critical dimension scanning electron microscope (CDSEM). The problem with these techniques is that it is both time consuming and is not timely in catching the problem when it starts to occur. Additionally, they only detect the problem when it is relatively severe, while it may begin to have an electrical impact on performance or yield prior to this point.